A circuit, which controls a current value flowing to a switching device through turning on and off this switching device, includes, for example, a boosting chopper circuit employing an active-filter as shown in FIG. 6 and a half-bridge type HID lighting circuit as shown in FIG. 7. In this kind of circuit, it is necessary to detect the current flowing to the switching device and to perform on-off control of the switching device based on the detected current value. Therefore, it becomes the matter how the current flowing to the switching device is detected.
Namely, according to a boosting chopper circuit employing the active-filter as shown in FIG. 6, a DC power supply (84) is constituted by an AC power supply (81) and a full-wave rectifying circuit (82), a choke coil (85) and a switching device (semiconductor switch) (86) are connected in series between output-terminals of the DC power supply (84), and a series circuit of a diode (87) and a capacitor (88) is connected to the both ends of the switching device (86) in parallel, so that electromagnetic energy is accumulated to the choke coil (85) by performing the on-off operation of the switching device (86) and emitted into the capacitor (88) through the diode (87), thereby accumulating a higher voltage than a voltage obtained by rectifying the AC power supply (81) through the full-wave rectifying circuit (82) to the capacitor (88).
A switch control circuit (89) detects the current flowing to the switching device (86) through a resistor (for example, 1 ohm) (90), produces an output of the ON signal (S), which is high voltage (H), to the switching device (86), brings the switching device (86) to be turned on while producing the output of the ON signal (S) (the output signal is becoming to high voltage (H)), and brings the switching device (86) to be turned off by shutting off the ON signal (S) (the output signal is becoming to low voltage (L)).
And, the switch control circuit (89) starts producing an output of the ON signal (S) (turning on the switching device (86)) when the current of choke coil (85) and the diode (87) becomes 0, and shuts off the ON signal (S) (turns off the switching device (86)) when the current flowing to the switching device (86) detected through the resistor (90) reaches the threshold corresponding to the peak value of the current of the desired choke coil (85). Therefore, stable DC voltage is obtained on the side of the capacitor (88) through performing on-off control in high frequency on the switching device (86).
Moreover, according to a half-bridge type HID lighting circuit as shown in FIG. 7, a series circuit of a pair of switching devises (92, 93) is connected between output-terminals of a DC power supply (91) and a series circuit of a pair of capacitors (94, 95) is also connected between them, and a choke coil (97), a HID lamp (98), a choke coil (99) and a capacitor (100) are connected between a midpoint of the pair of switching devices (92, 93) and a midpoint of the pair of capacitor (94, 95). Additionally, a resistor (103) (for example, 1 ohm) is connected between the capacitor (100) and the midpoint of the pair of capacitors (94, 95). The HID lamp (98) is lit up by making the pair of switching device (92, 93) perform on-off operation in an alternating sequence through a switch control circuit (104) based on the current flowing to the pair of switching devices (92, 93) detected by the resistor (103).
However, according to the conventional current detecting circuit as shown in FIG. 6, the resistor (90) connected with the switching device (86) in series detects the current flowing to the switching device (86). Therefore, there are problems that false detection and power loss might occur because spike-like noise is developed by the inductance content of the resistor detecting the current flowing to the switching device (86).
More specifically, where the current flowing to the switching device (86) is set to I, the voltage between the both ends of resistor (90) is set to V, and the resistance of the resistor (90) is set to R, the current flowing to the switching device (86) is detected as the voltage according to the V=IR relation. Therefore, the detecting voltage is enabled to be higher by increasing voltage (resistance R). However, power loss will occur in the resistor (90) because of the relation of W=IV where the power loss in the resistor (90) is set to W. Although the power loss in the resistor (90) is reduced by decreasing the voltage (resistance R), the detecting voltage becomes lower. Therefore, it becomes difficult to detect the detecting voltage with high accuracy because of the noise when the noise level is high.
Moreover, according to the conventional current detecting circuit as shown in FIG. 7, since the current flowing to the pair of switching devices (92, 93) is detected through the resistor (103), in addition to the similar problems as the case of the circuit in FIG. 6, it is necessary to provide a current detecting circuit (105) with such as a comparator circuit (106), and to insulate between the current detecting circuit (105) and the switch control circuit (104) through the use of, for example, a photocoupler (108) to carry the current detecting signal to the switch control circuit (104).
In addition, according to the method of detecting current flowing to the switch device as shown in FIG. 8, it is possible to detect the current at both ends of a resistor (112) provided to a secondary coil (111b) of a current transformer (111) which is constituted by a primary coil (111a) and the secondary coil (111b). In this case, although it is possible to detect the current with high voltage converted from small current, with low power loss, and in an insulated form by making more number of turns of secondary coil (111b) than those of primary coil (111a), there is a problem that it is impossible to detect the DC content.
Moreover, as shown in FIG. 9, it is possible to detect the current through a resistor (114) provided to the switching device in series, and to multiply the voltage through a transformer (115). Although this circuit has similar effects as the circuit shown in FIG. 8, there is a problem that it is impossible to detect the DC content.
Furthermore, although a method of detecting DC content by use of a hall device has been in practical use, there is a problem that the cost of this case becomes more expensive due to a lot of components used in the circuit.
Therefore, in view of the above-mentioned problems, an object of this invention is to provide a current detecting circuit, in which the current flowing to a switching device is enabled to be detected with high accuracy, with low power loss, with high voltage, and in an insulated form. Patent reference 1: JP,7-231650, A